1. Technical Field
The present invention relates in general to magnetoresistive (MR) read sensors or heads for magnetic recording systems and, in particular, to an improved system, method, and apparatus for such sensors that operate with the current-perpendicular-to-the-plane mode.
2. Description of the Related Art
In certain types of MR read sensors or heads for magnetic recording systems, the sense current passes perpendicularly through the planes of the layers making up the sensor. Such sensors are called “current-perpendicular-to-the-plane” or CPP sensors. CPP sensors are distinguished from “spin-valve” type MR sensors widely used in commercially available magnetic recording disk drives because spin-valve sensors operate with the sense “current-in-the-plane” of the sensor layers, or in CIP mode.
One type of CPP sensor is a magnetic tunnel junction (MTJ) sensor comprising two ferromagnetic layers separated by a thin insulating tunnel barrier layer and is based on the phenomenon of spin-polarized electron tunneling. The response of a MTJ sensor is determined by measuring the resistance of the MTJ when a sense current is passed perpendicularly through the MTJ from one ferromagnetic layer to the other. The probability of tunneling of charge carriers across the insulating tunnel barrier layer depends on the relative alignment of the magnetic moments (magnetization directions) of the two ferromagnetic layers. In addition to MTJ sensors, giant magnetoresistive (GMR) type MR sensors have also been proposed to operate in the CPP mode.
One of the problems with CPP MTJ and GMR sensors is the ability to generate an output signal that is both stable and linear with the magnetic field strength from the recorded medium. The sensing ferromagnetic layer in the CPP sensor must be stabilized to avoid magnetic instabilities and degradation of the signal-to-noise performance of the sensor by hysteresis. The problem of sensor stabilization using a conventional tail stabilization approach is especially difficult in the case of a CPP sensor. For example, with an MTJ MR read head, the sense current passes perpendicularly through the ferromagnetic layers and the tunnel barrier layer. Thus, any hard bias metallic materials used in the tails to stabilize the sensing ferromagnetic layer will short circuit the electrical resistance of the MTJ if they come in contact with the ferromagnetic layers. This can be solved with a thin insulation layer as shown in U.S. Pat. No. 5,729,410, which is incorporated herein by reference. However, that solution is dependent on the spacing from the sensor to the hard bias layer and the shape of the hard bias layer.
One type of MTJ MR read head has a biasing ferromagnetic layer magnetostatically coupled with the sensing ferromagnetic layer of the MTJ to provide longitudinal bias to the sensing ferromagnetic layer. As shown in FIG. 1, this MTJ MR head is a sensor structure made up of a stack of layers formed between a bottom shield 10 and a top shield 12, the shields being typically formed of relatively thick highly magnetically permeable material, such as permalloy. The shields 10, 12 have generally planar surfaces spaced apart by a gap 53. The gap material 50, 52 on the sides of the sensor structure is an insulating material, typically an oxide such as alumina. The layers in the stack are a bottom electrical lead 20, the MTJ sensor 30, the longitudinal bias stack 40, and top electrical lead 22.
The MTJ sensor 30 is made up of an antiferromagnetic layer 32, a fixed ferromagnetic layer 34 exchange biased with the antiferromagnetic layer 32 so that its magnetic moment cannot rotate in the presence of an applied magnetic field, an insulating tunnel barrier layer 36 in contact with the fixed ferromagnetic layer 34, and a sensing or “free” ferromagnetic layer 38 in contact with the tunnel barrier layer 36 and whose magnetic moment is free to rotate in the presence of an applied magnetic field. The longitudinal bias stack 40 includes a nonmagnetic electrically conductive spacer layer 42, a biasing ferromagnetic layer 44 that has its magnetic moment aligned generally within the plane of the device and is separated from the ferromagnetic layer 38 by the spacer layer 42, and an optional antiferromagnetic layer 46 exchange coupled to the biasing ferromagnetic layer 44. The self field or demagnetizing field from the biasing ferromagnetic layer 44 magnetostatically couples with the edges of the sensing ferromagnetic layer 38 to stabilize its magnetic moment, and, to linearize the output of the device. The electrically conductive spacer layer 42 prevents direct exchange coupling between the biasing ferromagnetic layer 44 and the sensing ferromagnetic layer 38 in the MTJ sensor 30 and allows sense current to flow perpendicularly through the layers in the stack between the two leads 20, 22.
As shown in FIG. 1, the width of the data tracks of the recorded media is determined by the trackwidth (TW) of the MR sensor. The shielding geometry provided by shields 10, 12 of the MR sensor attenuates the flux coming from adjacent magnetic transitions of the recorded media along the downtrack direction (perpendicular to the layers in the stack) and therefore enhances the sensor's linear resolution. However, for very small trackwidths this shielding geometry does not provide adequate suppression of side reading caused by flux coming from adjacent tracks. Thus, an improved design for an MR sensor that overcomes these limitations of the prior art would be desirable.